Contemporary circuit bearing substrates comprise those based on glass epoxy printed circuit boards, polyimide based flexible substrates, paper based printed circuit boards, polymer based substrates, and alumina based hybrid circuits. All of these technologies are employed to attach and interconnect various discrete electrical components to form a complete module or sub-assembly.
Miniaturization requirements drive these packaging technologies to resort to two-sided and multi-layer implementations. Because of this, these circuits require not only the traditional planar interconnect structures but also inter-layer and inter-side electrical connections.
FIG. 1 illustrates a prior art method for constructing a via between multiple layers, in this case two outer sides of an alumina substrate. This is accomplished by using an off-contact printing process. This process is limited to providing small area, or small cross-sectional area vias, has many steps making it costly to manufacture, and is prone to poor manufacturing yields.
Generally, a bottom surface 101 of an alumina substrate 103 is disposed onto a top surface of a fixture 105. A through-hole, or via 107 is provided through the alumina substrate 103. This via 107 may be die cut when the alumina substrate 103 is green, or laser drilled after the green alumina is fired. A stainless steel mesh screen, with selective masking in the form of an emulsion, 109 is positioned just above a top surface of the alumina substrate 103. Two apertures 113, 115 are positioned aligned over the via 107. These apertures represent areas of the screen 109 without the emulsion masking. A centered area 117 is shown directly above the center of the via 107 and is employed as a mask area. Without this centered mask area 117 a large blob of ink 119 would adhere to the bottom side of the screen 109 proximate the apertures 113 and 115. If it does, then the ink would smear when the screen 109 is later removed. This smearing is commonly referred to as back side splashing. This will cause problems as described later.
In a first step, a conductive material 119, typically a thick-film type ink is disposed on a top surface 121 of the screen 109. A squeegee 123 is utilized to distribute a portion of the ink 119 into the via 107 as the squeegee 123 transits in a direction indicated by reference number 125.
As the process proceeds, as shown in step 2, the squeegee 123 moves, and the ink 119 gets pushed across the top surface 121 of the screen 109. Notice that the screen starts to deform and move toward the top side 111 of the alumina substrate 103 as a result of the pressure applied to the squeegee 123.
Then, in step 3, the ink 119 gets forced into the aperture 113 through the action of the squeegee 123 and gravity. As the process continues, in step 4 the ink 119 gets forced into the aperture 115.
Then, in step 5 the squeegee is removed, and the screen 109 springs back to its original position. Note that the ink 119 is now positioned at the top of the via 107. As mentioned above, without the centered mask area 117, a large blob of ink would form on the bottom side of the screen 109. If this was allowed to happen, then this blob of ink could form a bridge between the bottom of the screen 109 and the top edge of the via 107. When the screen 109 is pulled away from the alumina substrate 103 this blob would smear onto the top side 111 of the alumina substrate 103. This is undesirable and would cause manufacturing yield problems. The centered mask area 117 is therefore required in an off-contact printing process. Detriments of the centered mask area 117 will be described later.
Next, in step 6, after the screen 109 is removed, a vacuum is drawn, as shown by the arrow 127, through the via 107. As a result the ink 119 is drawn downward along an inner wall 129 of the via 107. Because the amount of ink was necessarily limited by the centered mask area 117 there is an insufficient amount to transition completely down the inner wall 129 of the via 107 to the bottom of the alumina substrate 103. This is the primary detriments of the centered mask area 117. Note that the ink 119 is not shown on the face of the inner wall 129 of the via 107 for illustration clarity only.
Because there is an insufficient amount of ink 119 to transition completely down the inner wall 129 of the via 107 to the bottom of the alumina substrate 103, the first six steps of the process must be repeated with the alumina substrate 102 inverted. These process steps are shown in FIG. 1 in steps 7 through 12.
Even though this lengthy process will work for vias having small cross-sections, it will not work for vias having large cross-sections such as those used to carry heavy currents.
FIG. 2 shows one prior art method for creating a relatively large oblong via with a small cross-sectional area. Reference number 201 represents a substrate and reference number 203 shows a sectioned elongated via. This type of via is difficult and expensive to fabricate in a production environment because tooling for green tape punching is impractical to fabricate for an alumina substrate. In this case one must resort to laser drilling which is much more expensive.
What is needed is an improved method for fabricating a circuit element through a substrate. The improved method should have fewer steps and allow for easy to fabricate large cross-sectional geometries for vias.